A multiband resonator having multiple resonance frequency bands described in Non-Patent Literature 1 (S. Kitazawa, H. Miyake, M. Geshiro, and M. Ohashi, “A Multilayer Single-Input/Single-Output Dual-Band Filter Fabricated in a High Permittivity LTCC Substrate”, IEICE Transactions on Electronics, Vol. E89-C, No. 4, pp. 509-516, April 2006) is known as the conventional art. The multiband resonator has a structure in which two resonators, each having one resonance frequency band that differs from the other, are stacked with a shield layer between them. The resonators are coupled to input and output ports to form a multiband resonator. A wide variety of resonators with single resonance frequency band have been disclosed, such as the one described in Non-Patent Literature 2 (Kentaro Yoshida, Toshiaki Kitamura, Masahiro Geshiro, and Toshio Ishizaki, “Study on Characteristics of Folded Comb-Line Filter”, IEICE Transactions C, Vol. J86-C, No. 4, pp. 442-449, April 2003).
In the conventional art, resonators having different resonance frequency bands are stacked with a shield between them, in order to provide multiple resonance frequency bands. That is, independent resonators are combined together in such a manner that the resonators do not affect each other. Accordingly, such multiband resonators require many layers and therefore are difficult to reduce in size.
An object of the present invention is to reduce the size of a multiband resonator having multiple resonance frequency bands.